Data contained in documents that has been scanned at high resolutions requires very large amounts of storage space. This data is typically subjected to some form of data compression in order to avoid the high costs that would be associated with storing it. "Lossless" compression methods such as Lempel-Ziv Welch (LZW) do not perform particularly well on portions of the document that are scanned pixel maps; "lossy" methods such as JPEG work fairly well on continuous-tone pixel maps, but they do not work particularly well on the parts of the document that contain text. To optimize image data compression, techniques, which can recognize the type of data being compressed, are needed.
One approach to satisfy the compression needs of differing types of data has been to use Mixed Raster Content (MRC) which involves separating a composite image--one having text intermingled with color or gray scale information--into three planes, and separately applying an appropriate compression technique to each plane. An approach such as this is discussed in U.S. Pat. No. 5,778,092 to MacLeod et al. issued Jul. 7, 1998, which discloses a technique for compressing a color or gray scale pixel map that represents a document. The pixel map is decomposed into a three-plane representation--a reduced-resolution foreground plane, a reduced-resolution background plane, and a high-resolution binary selector plane. The foreground plane contains the color or gray scale information of foreground items such as text. The background plane contains the color or gray scale information for the "backgrounds" of the page and the continuous tone pictures that are contained on the page. The selector plane stores information for selecting from either the foreground plane or background plane during decompression.
While the MRC technique has shown to be successful at separately processing planes, the segmentation process leaves data in both planes in the areas that will not be chosen by the selector plane. This often causes an increase in the number of bits that are required to encode the entire image, thereby decreasing its compression ratio. This results in inconveniences to the user of a printer, fax machine, scanner or other device in which the technique has been incorporated. For this reason, it is advantageous to somehow reduce the amount of data residing on each plane prior to processing. The present invention is directed to using the information that is contained in the selector plane to aid in reducing the amount of data residing on the foreground and/or background planes. More specifically, the invention takes advantage of the fact that when the selector plane designates a plane to provide information about a given pixel, the information on the other plane that pertains to the same pixel will not be used. The invention provides improved compression of the multi-plane image by treating this useless data in the described manner.
The following disclosures may be relevant to aspects of the present invention:
U.S. Pat. No. 5,251,271 to Fling issued Oct. 5, 1993 discloses a method for registering digitized multiplane color images. The method designates one plane as the reference plane and registers each of the other warped planes with the reference plane. Each plane comprises pixels representing luminosity values having scalar x and y coordinates representing positions in the horizontal and vertical directions, respectively, of the plane. The planes are divided into regions. Correlation values are calculated for regions within the divisional region of the reference plane with a plurality of regions offset from the corresponding warped divisional region. A warp error value is calculated for each pixel of each divisional region as a function of the scalar offset. The warp error values are interpolated and added to the current position of each pixel of the warped plane.
Separate processing of various types of data contained in a document is disclosed in U.S. Pat. No. 5,060,980 to Johnson et al. issued Oct. 29, 1991 which describes a "forms" that includes user modifiable fields and an encoded description of the location, size, type, etc. of the fields to allow for direct programming of a form interpreter. Other information including the processing of the form, encoded data, etc. may be included in the encoded information. A system for creating forms carrying an encoded description of selected attributes of the fields includes means for selecting or creating fields and locating the fields on a form while generating, substantially simultaneously, the encoded description of the selected attributes. A form composer then allows merging of the form and its encoded description for printing or electronic transmission. A system for reading such forms includes a scanner, decoding device, and processor. By reading such forms, data may be entered into or recalled from a data processing system, or a form interpreter may be programmed, locally or remotely, for subsequent handling of forms.
U.S. Pat. No. 5,784,175 to Lee, issued Jul. 21, 1998 discloses a video compression encoder process for compressing digitized video signals representing display motion in video sequences of multiple image frames. The encoder process utilizes object-based video compression to improve the accuracy and versatility of encoding interframe motion and intraframe image features. Video information is compressed relative to objects of arbitrary configurations, rather than fixed, regular arrays of pixels as in conventional video compression methods. This reduces the error components and thereby improves the compression efficiency and accuracy. As another benefit, object-based video compression of this invention provides interactive video editing capabilities for processing compressed video information.
U.S. Pat. No. 5,303,313 to Mark et al. issued Apr. 12, 1994 describes image compression based on symbol matching. An image is "pre-compressed" prior to symbol matching using run length encoding. Symbols are then extracted from the run length representation. A voting scheme is used in conjunction with a plurality of similarity tests to improve symbol matching accuracy. A template composition scheme wherein the template may be modified based on symbol matches is also disclosed.
Concurrently filed U.S. patent application by DeQueiroz et al. identified as Ser. No. 09/206,487 entitled "Method and Apparatus for Pre-processing Mixed Raster Content Planes to Improve the Quality of a Decompressed Image and Increase Document Compression Ratios" and assigned to the assignee of the present invention discloses a technique for processing a color or gray scale pixel map representing a document is disclosed. The pixel map is decomposed into a three-plane representation, a reduced-resolution "upper" plane, a reduced-resolution "lower" plane, and a high-resolution binary selector plane. The "upper" and "lower" planes contain the color or gray scale for the page as well as the continuous tone pictures that are contained on the page. The selector plane stores information for selecting from either the foreground plane or background plane during decompression. Information contained in the selector plane is first used to pre-process the upper and lower planes to reduce the amount of data on each of the other two planes that will be subjected to further processing. Each of the pre-processed planes is compressed using a compression technique optimal for the type of data that resides upon it.
All of the references cited herein are incorporated by reference for their teachings.
Accordingly, although known apparatus and processes are suitable for their intended purposes, a need remains for a method and apparatus that can efficiently process digital image data by separately compressing the various portions of a composite image.